When you run smbd -V on your Snow Leopard installation, you'll see it's running SAMBA version 3.0.28a-apple. While I'm not sure how much difference the "-apple" makes, version 3.0.28a is old. Very old. In other words, it's riddled with bugs. Apple hasn't updated SAMBA in 3 years, and for Lion, they're dumping it altogether for something homegrown. The reason? SAMBA is now GPLv3.

There is no such thing as a free lunch - anything in life will always come with some gotcha, some sort of requirement to give up something to gain something. Apple has decided, based on a number of reasons (licensing being one of them) to write their own in house SMB implementation. Although GPL3 maybe a hugely obvious reason I am sure that technical reasoning is probably more likely the motivation behind it.